Recently, wireless communications systems, such as the Radio Frequency Identification (RFID) and a wireless sensor net work, have been attracting attention. Such systems less frequently (at several hundred milliseconds to several hours' intervals) transmit and receive a small amount of data. Designed small and powered by a battery, a wireless communications apparatus used in such wireless communications systems has to have a long operational lifetime (several months to several years). For the wireless communications apparatus, the standby time for reception consists of most of the operating time. Hence, the wireless communications apparatus has to be operated on ultralow power.
There is a technique introducing a combination of a power-saving wireless apparatus for wake-up and a power saving wireless apparatus for data communication in order to reduce power consumption during the standby time for reception. Patent Literature 1, for example, discloses a technique to utilize a frequency Fa for wake-up and a frequency Fg for data communication, and a filter separating the frequencies with each other. When the wake-up signal is detected, a wireless unit for data communication is activated.
Furthermore, Patent Literature 2 discloses a technique to employ an ON-OFF Keying (OOK) modulating signal, which requires little power for the transmission and reception, so as to notify a communications partner of a frequency channel to be used for the data communication. The technique in Patent Literature 2 shows the following: wake-up signals are OOK-modulated, and each of the OOK-modulated wake-up signals is transmitted simultaneously over a corresponding one of frequencies which are different with each other, the transmitted signals are demodulated for each of the frequencies at the reception side, and the data communication is established using the frequency with which the wake-up signal is successfully received.